Idiopathic pulmonary arterial hypertension (IPAH) is a life-threatening disease of the pulmonary circulation affecting children and adults. Accumulating evidence indicates that one key factor that contributes to the pathogenesis of vascular diseases, including PAH, is an increase in reactive oxygen species, such as superoxide (O2-), that exceed antioxidant capabilities. One critically important antioxidant in the vessel wall is extracellular superoxide dismutase (EC-SOD), the sole enzymatic defense against extracellular O2-. EC-SOD is the most highly expressed SOD isoform in the vasculature under normal conditions, and EC-SOD expression is decreased in animal models of lung or vascular injury and several human diseases, including one study showing a decrease in EC-SOD protein in the bronchus of 8 patients with IPAH. Augmenting EC- SOD activity in transgenic mice or through adenoviral gene delivery attenuates pulmonary vascular remodeling and pulmonary hypertension. The regulation of EC-SOD in IPAH has not been investigated. It is now recognized that gene expression can be regulated by epigenetic modifications including cytosine methylation within the promoter region, specifically cytosines adjacent to guanosine nucleotides (CpG islands). New data identify CpG islands within the EC-SOD promoter that can be methylated, and indicate that hypermethylation of the EC-SOD promoter inhibits EC-SOD transcription in human cancer cells, contributing to enhanced tumor cell proliferation. Based on these data, we hypothesize that DNA methylation of the EC-SOD promoter mediates silencing of this protective gene and contributes to the pathogenesis of idiopathic pulmonary arterial hypertension. Aim 1 will utilize lung and pulmonary artery tissue and serum procured at the time of organ transplantation in patients with IPAH, disease-specific controls with PAH due to other causes, or patients without PAH to determine whether EC-SOD gene and protein as well as enzyme activity are decreased in IPAH and test the impact of EC-SOD expression on pulmonary artery smooth muscle cell proliferation. Specific Aim 2 will then use bisulfite genomic sequencing to test whether the EC-SOD promoter in pulmonary artery tissue and pulmonary artery smooth muscle cells from humans with IPAH is hypermethylated, and if reversal of methylation restores EC-SOD expression and normal cell proliferation. The tissue and cell samples used in this study will have been procured and processed through the efforts of the investigators of the Pulmonary Hypertension Breakthrough Initiative. Our findings will serve as strong preliminary data for the subsequent submission of a comprehensive RO1 application investigating the regulation of EC-SOD in IPAH. The long term goals are to establish new pathways important in the regulation of this pivotal antioxidant enzyme in the vessel wall, identify the role of extracellular superoxide in proliferation, inflammation and fibrosis contributing to pulmonary vascular remodeling, and provide a rationale for novel therapeutic tools to improve treatment of this lethal disease affecting children and adults.